Time dependence dilatometry

The glassy state is, thermodynamically, a nonequilibrium state. Thus, the glass transition is a molecular relaxation process having a time dependent nature. The glass transition behavior of various types of lignins has mainly been investigated by dilatometry [14], differential scanning calorimetry (DSC) [21,22,25], viscoelastic measurements [18,19] and broad-line NMR [17,35]. 

Other thermal analysis techniques such as dilatometry in Sect. 4.1 or thermo-mechanical analysis in Sect. 4.5 can also be used to study the time dependence of T. Especially suited for measurement of the frequency response are dynamic mechanic analyses in Sects. 4.5.4 and 4.5.5, and dielectric thermal analyses in Sect. 4.5.6. Although the different techniques respond to different external excitations, the obtained relaxation times are similar, as shown in Fig. 6.117. Over wider temperature... 

The technique of loading dilatometry, in which a small, controlled uniaxial stress Pj is applied to a powda- compact during sintering has been used to investigate the simultaneous occurrence of densification and creep, as well as their interaction (57,58). Parameters such as the stress intensity factor < > and the sintering stress 2 can be determined from the data. In the experiments, simultaneous measurement of the time-dependent axial and radial strains allows the determination of the... 

Fig. 5.24. sPP Crystallization isotherms as given by the time dependence of P (from SAXS, filled symbols) and of the density change dp (from dilatometry, open symbols). The initial slope indicates a kinetic power law P [Pg.186]

The decrease in volume that accompanies physical aging is known as volume recovery or volume relaxation. Dilatometry (Dil) can be used to follow the volume relaxation in glasses by monitoring the time-dependence of the volume change on aging. The material is either cooled from above Tg to the aging temperature 7], (down-jump) and the isothermal volume contraction is measured or the sample is heated in the glassy state (up-jump), in which case an expansion follows. 

Figure 35. Glass transition temperature 7., (determined by dilatometry) and relaxation time r at 131°C as a function of annealing time in air at 180°C for a film thickness of 63 nm. The dotted lines serve as a guide for the reader. Inset Dilatometric determination of the glass transition temperature. Upper. Normalized capacitance Cn0nn versus temperature at 106Hz (the solid lines represent linear dependencies, the dotted line marks the position of the glass transition temperature). Lower. The corresponding first and second numerical derivatives of Cnonn (in arbitrary units) as a function of temperature.

Pioneering work of Tool [1946, 1948] on inorganic glasses using dilatometry indicated that volume relaxation after a temperature jump from an initial equilibrium state could not be described simply by a kinetic model in which the relaxation time T was solely dependent on the temperature. Tool therefore proposed that r was also a function of the structure of the glass, and this led to the definition of the Active temperature Tf. 

A11 transition temperatures depend on factors such as sample MW and conditions of measurement. Note that a-, fi-, and mesophase transition values depend upon the measurement method, molecular weight, and specimen history. Consult references for the techniques employed. For example, some authors claim (with good reason) that dynamical techniques are only related to classical dilatometry (l min ) results. Logically, all comparisons should be on similar time scales. 

Results may be quoted in terms of seconds or minutes, depending upon the crystallization rate. Crystallization rates involving half-times in excess of a few minntes are rarely of interest to commercial enterprises. The determination of half-times in excess of an hour with a differential scanning calorimeter is not very practical due to the difficulty of measuring the very low heat transfer rates involved if such measurements are desired, dilatometry is a more practical alternative. 

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